Hydraulic dampers, in the field also referred to as buffers, are commonly installed in central buffer couplings adapted for connection of rail vehicles. In the central buffer coupling a damper can be effective for absorbing shock loads in both compression and extension of the damper, in this way reducing jerking and smoothening the ride for passengers.
The general function and structure of the subject hydraulic dampers include a hollow piston which is received axially movable in a cylindrical housing. A volume of hydraulic fluid is contained in a working chamber in the housing. The working chamber communicates with an overflow chamber in the piston via a restriction. In compression of the damper, such as in the case of a buff load higher than moderate which pushes the piston further into the housing, hydraulic fluid is forced via the restriction into the overflow chamber as the volume of the working chamber is reduced. A partitioning element which slides freely in the hollow piston is displaced by the inrushing fluid, this way increasing the volume of the overflow chamber. The partition wall is displaced against the force of a compressible spring which is loaded while absorbing most or all of the energy that caused compression of the damper. This spring is usually a gas volume which under moderate load absorbs the energy generated in compression of the damper. In extension of the damper the spring releases its accommodated energy to return hydraulic fluid in the overflow chamber back to the working chamber. The reverse flow is typically routed other way bypassing the restriction, this way permitting a non-restricted return of the piston to its unloaded position. With the purpose of avoiding a heavy recoil as the piston is returned in extension motion, an additional chamber can be arranged to receive a smaller volume of hydraulic fluid during compression while returning the same volume via a restricted passage during extension of the damper, this way balancing the expansion of the gas spring and of the damper.
Dampers of the type are previously known in the literature. In EP1352802B1 there is disclosed a damper for a central buffer coupling arrangement wherein a balancing chamber is located in an annular space defined between a cylinder and a piston received movable in the cylinder. An asymmetric throttle means for fluid flow into the balancing chamber is located in the flow path which connects a working chamber in the cylinder with an overflow chamber in the piston, the asymmetric throttle means in other words being arranged in-line with the restriction that controls the flow from the working chamber to the overflow chamber. The asymmetric throttle means includes radially formed inlet and outlet bores, a non-return valve in the inlet bore and a reduced diameter of the outlet bore.
In DE1455227A1 there is disclosed a gas-hydraulic damper arrangement wherein a balancing chamber is arranged in an annular space that is formed between a piston and a cylinder in which the piston is accommodated. The balancing chamber communicates with a working chamber via an axial bore through an over-sized end-plate covering the end of the piston. A throttling effect to the return flow from the balancing chamber to the working chamber is achieved by means of a free-floating ring in the balancing chamber, the ring during a return stroke being moved towards the end plate by the pressure differential between the balancing chamber and the working chamber. An outlet bore in the ring is positioned in mating relation with the inlet bore of same diameter through the end-plate of the piston, thus extending the flow path for the hydraulic fluid in the return-flow.
U.S. Pat. No. 4,973,854A discloses a vibration damper comprising two working chambers, one of which is arranged in the cylinder and the other of which is located in an annular space defined between the cylinder and a piston movable in the cylinder. The annular chamber communicates with the working chamber in the cylinder via inlet/outlet bores through a radial flange formed in the inner end of the piston. The vibration damper is shown with one-way valves installed in the bores. Throttle means in the bores are mentioned in the document, but are not particularly described or shown in the drawings.